Most people eventually heal, CNN reports, but between 8 percent and 15 percent are likely to develop post-traumatic stress disorder, according to Russell T. Jones, a professor of psychology at Virginia Tech who counseled survivors of that mass shooting.

Children with PTSD typically experience three types of symptoms: re-experiencing the trauma, avoidance and increased agitation.

Children ages 5 to 12 with PTSD may not have flashbacks or problems remembering parts of the trauma, the way adults experience the disorder. Instead, they may show signs in their play, according to the National Center for PTSD.

They might keep repeating a part of the trauma. These games do not make their worry and distress go away. For example, a child might always want to play shooting games after he sees a school shooting. Children may also fit parts of the trauma into their daily lives. For example, a child might carry a gun to school after seeing a school shooting.

Dr. Melissa Brymer, director of terrorism and disaster programs with the National Center for Child Traumatic Stress, has studied incidents of post-traumatic stress disorder after school shootings in Southern California in 2001 and at Virginia Tech in 2007, according to NBC.

A study from a shooting at Santana High School in Santee, Calif., showed close to one-fourth of the 247 students directly exposed suffered from PTSD or partial PTSD eight to nine months after incident, according to Pediatric News.

Among all 1,160 students screened, 4.9 precent met criteria for PTSD, and 12.5 percent met partial criteria for PTSD. Depression was present in 15.4 percent of all students and 18.7 percent of those with direct exposure.